How should you sit when practising?

I tend to sit on the edge of my sofa or chair and with both my feet flat on the floor rest the body of the bass on my right thigh. However, I think this is contributing to a twinging sharp pain up and to the left of my right shoulder-blade. My friend plays guitar and sits in the same manner but rests the body of his instrument between his legs, with the length of a bass neck, I find this uncomfortable. What do you find the most comfortable position is?

I like the bass to be in the same relative position when sitting or standing. I sit on a bar stool and let my right leg extend fairly straight. I also spend some of my practice time standing. I think if your bass position varies greatly from sitting to standing it may cause you some problems.

I still get a twinging pain in my back, and neither do I feel relaxed when playing sat down with the strap, it still doesn't hang right. When I'm stood up it's OK, but sometimes I just enjoy sitting. I want comfort to play, comfort to sit and a relaxed posture. Thanks anyway, I'm gonna try shortening my strap further.

If you wear your bass high or moderately low (this works as long as your bass isn't touching your thigh while standing), here's a great idea:

I use one of those strange looking "ergonomically correct" computer chairs. For those of you unfamiliar, I'll try to give a description, as best I can. instead of just sitting upright as in a standard garden-variety chair, you kneel on a pad and also rest your butt on a pad, splitting your weight between the two. This tends to make you sit more straight up, and eases stress on your back. It also has the added advantage for those of us strap instrument players of letting you "sit" but still have the instrument hang in the same position as standing.

As MJB and Bruce pointed out, it's best to practice with the bass in the same position that it'll be in when performing. This type of chair will allow most players to do so while still sitting, and with the added benefit of almost forcing you to maintain proper posture while doing so.

You can find these chairs at most office supply stores (i.e. Office Depot, Office Max, Staples in the US), they cost a bit (the one I'm using now cost me $60) but if you get a good one, they'll last a good long while. The one I've got now, I've had for about 6 or 7 years. I recommend the tubular steel ones over the less expensive wooden ones, they seem to hold up better (my first, wooden one, broke after about 4 years).

APouncer, something else to keep in mind is this. For many years I'd "hunch up" while playing. Y'know shoulders sort of folded in and forward, back not straight. This lead to some pain and discomfort after a while. Try sitting straight up (as this chair tends to make you) and keeping your shoulders straight, that may relieve your discomfort.

Gard, it's working already - sitting up straight! My friends suggested that I really look at myself whilst playing, and although I have been playing instruments for 10 years (bass for 2), bad habits had crept back in - holding my breath during difficult passages, hunching when new to a piece - all the training I had not to do this when I was 15 has been forgotten. I've also ordered a stool like that! Cheers for all the advice guys, and have a great weekend! Rock over London, rock on Chicago!